Which of these numbers is prime? ${15,\ 37,\ 42,\ 63,\ 77}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 37 are 1 and 37. The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 77 are 1, 7, 11, and 77. Thus, 37 is a prime number.